The photocatalytic properties of the semiconductor material titanium dioxide result from the promotion of electrons from the valence band to the conduction band under the influence of ultraviolet (UV) and near-UV radiation. The reactive electron-hole pairs that are created migrate to the surface of the titanium dioxide particles where the holes oxidize adsorbed water to produce reactive hydroxyl radicals and the electrons reduce adsorbed oxygen to produce superoxide radicals, both of which can degrade NOx and volatile organic compounds (VOCs) in the air. In view of these properties, photocatalytic titanium dioxide has been employed in coatings and the like to remove pollutants from the air. Such coatings may also have the advantage of being self-cleaning since soil (grease, mildew, mold, algae, etc.) is also oxidized on the surface.
In many applications, it is desirable for the titanium dioxide coating to be transparent in order to maintain the original appearance of the substrate (e.g., ceramic tile, paving block, siding, etc.) or its original transparency (e.g., window glass, car windshield, etc.). Titanium dioxide colloidal sols have proven to be a useful precursor material for forming such transparent coatings.
For example, U.S. Pat. No. 6,420,437 to Mori et al, which is hereby incorporated by reference, discloses a neutral titanium dioxide colloid sol said to have high stability in a neutral range and which is capable of forming a colorless transparent coating even when dried at room temperature. The sol is produced by mixing an acid titanium dioxide sol comprising 50 to 100 parts by weight of titanium dioxide colloidal particles and 5 to 50 parts by weight of a chelating agent for titanium ions with 1 to 50 parts by weight of an alkaline substance comprising at least one of alkali metal compounds and amine compounds, and optionally by adjusting the pH value of the liquid mixture to 5 to 10, or adjusting the pH value of the liquid mixture to 6 to 10, and then applying a deionization treatment to the mixture to thereby charge the titanium dioxide colloidal particles with negative electricity.
U.S. Pat. No. 6,627,336 to Ohmori et al., the disclosure of which is hereby incorporated by reference, describes an aqueous dispersion of titanium oxide particles, which are preferably predominantly comprised of brookite titanium oxide particles, comprising chloride ion, and a Brønsted base other than chloride ion, preferably nitrate ion and/or phosphate ion. The aqueous titanium oxide dispersion is prepared by hydrolyzed titanium tetrachloride in the presence of at least one kind of a Brønsted acid. Thin films formed from the aqueous titanium oxide dispersion are said to exhibit good photo-catalytic activity, transparency and adhesion to a base material.
U.S. Pat. No. 6,824,826 to Amadelli et al., the disclosure of which is hereby incorporated by reference, relates to the use of photocatalytic preparations of colloidal titanium dioxide optionally doped with a metal chosen from groups I-VA, and the lanthanide and actinide series of the periodic table, for preserving the original appearance of cementitious, stone, and marble products. Preparation of colloidal titanium dioxide by the controlled hydrolysis of titanium isopropoxide with nitric acid is provided in Example 1 of that patent.
U.S. Patent Pub. 2004/0241502 to Chung, Hoon et al., the disclosure of which is hereby incorporated by reference, relates to a method of manufacturing a neutral and transparent titanium dioxide colloid solution wherein titanium dioxide nano particles are dispersed, and a titanium dioxide colloid solution prepared by the method. In one method for preparing the sols, a titanium compound and a stabilizer are added to alcohol, neutralized by adding a basic solution, and then heated at a temperature of above 75° C. for more than 7 hours. The titanium compound can be, among others, tetraisopropanol titanium (titanium isopropoxide) and among the numerous stabilizers, glycolic acid is said to be suitable. The neutral titanium dioxide colloid solution prepared by the method are said to be stable and transparent.
Despite these advances in the art, there is considerable room for improvement as each of the known sols is not without its disadvantages. It is therefore an object of the invention to provide sols comprising photocatalytic titanium dioxide which are transparent. It is also and object of the invention to provide such transparent titanium dioxide sols which are stable over an extended period of time. It is a further object of the invention to provide novel methods for preparing such stable, transparent sols which are readily implemented on a commercial scale.
The foregoing discussion is presented solely to provide a better understanding of nature of the problems confronting the art and should not be construed in any way as an admission as to prior art nor should the citation of any reference herein be construed as an admission that such reference constitutes “prior art” to the instant application.